Solar concentrator

A solar concentrator concentrates the power of the sun onto a small area. This energy is harnessed in a variety of ways. For example, a system called SCARLET is currently flying on Deep Space I.

Designs

• Lenses. A variety of lenses designs are used to concentrate sunlight. A lens may be a simple piece of curved glass or plastic, or a complex Fresnel lens.

• Mirrors and reflectors. Fields of mirrors are used in some concentrated solar facilities on Earth. The individual mirrors rotate to track the sun. The construction can be made lightweight from polymers, e.g. Mylar and Hostephan. Computer simulations of radiation damage have shown that an aluminum covering of at least 0.1 mm thickness is necessary(reference).
• Fiber Optics

Utilization of Focused Solar Radiation on Mars

Total solar illumination on Mars is the sum of direct and diffuse illumination. Diffuse illumination cannot be focused, and this can significantly reduce the efficiency of solar reflectors on Mars. On average 50% of the light available on Mars is diffuse.(reference) So the surface of reflectors need to be 50% larger than what would be calculated with the total irradiation.

• Photovoltaics. Solar concentrators are often used in conjunction with solar panels to increase the panel's output. This maximizes the efficiency of a limited number of solar panels.

• Thermal Engines. The energy of the sun can boil liquids, causing changes in pressure. These pressure changes are harnessed by thermal engines. It is likely that liquids other than water will be used, due to the low temperatures on the surface. Carbon dioxide and carbon monoxide are both available as working fluids. Common thermal engine designs include the Sterling engine and the steam engine. Since the angular size of the sun as seen on Mars is smaller than it is on Earth, a larger mirror is needed to get the same sized image of the sun at about the same temperature as on Earth. Solar thermal engines should be capable of powering electric generators on Mars.

• Thermoelectric Generators. Thermoelectric materials convert heat into electricity.

• Photosynthesis. Greenhouses are a good target for solar concentration. The natural sunlight on Mars is less than half that on Earth.

• Lighting. Concentrated sunlight can be used to light settlements during the day.

Problems

Concentrated solar radiation can become hazardous in some situations.

• Radiation Damage. If solar radiation is concentrated for use in greenhouses or settlements, the harmful parts of the electromagnetic spectrum need to be filtered.

• Vision Damage. Looking directly into concentrated sunlight can damage vision.

• Heat Damage. Objects passing through the focus of a solar concentrator can be exposed to intense light and extremely high temperatures.
• Dust storm. During dust storms direct solar illumination may fall to nothing, while indirect illumination remains and can be used by photoelectric cells. Therefore in some cases photoelectric cells might be more appropriate as an energy source than thermoelectric generators than depend on direct illumination.

Maintenance effort

• The cosmic and solar radiation causes damage upon the concentrator mirrors. The particle bombardment causes blistering and foil carbonization. A lifetime of 10 years is assumed, thereafter the mirrors have to be replaced.
• Solar concentrators are susceptible to losses due to atmospheric diffraction and dust. During a dust storm concentrators lose most of their efficiency.
• Solar concentrators need to be cleaned to maintain their efficiency.

External links

• U.S. Department Of Defense: SCARLET
• NASA ADS: On the performance and lifetime of solar mirror foils in space

References

NASA Technical Memorandum 102299, Solar Radiation on Mars, Joseph Appelbaum and Dennis J. Flood,Lewis Research Center, Cleveland, Ohio, August 1989